Root/
1 | ROMFS - ROM FILE SYSTEM |
2 | |
3 | This is a quite dumb, read only filesystem, mainly for initial RAM |
4 | disks of installation disks. It has grown up by the need of having |
5 | modules linked at boot time. Using this filesystem, you get a very |
6 | similar feature, and even the possibility of a small kernel, with a |
7 | file system which doesn't take up useful memory from the router |
8 | functions in the basement of your office. |
9 | |
10 | For comparison, both the older minix and xiafs (the latter is now |
11 | defunct) filesystems, compiled as module need more than 20000 bytes, |
12 | while romfs is less than a page, about 4000 bytes (assuming i586 |
13 | code). Under the same conditions, the msdos filesystem would need |
14 | about 30K (and does not support device nodes or symlinks), while the |
15 | nfs module with nfsroot is about 57K. Furthermore, as a bit unfair |
16 | comparison, an actual rescue disk used up 3202 blocks with ext2, while |
17 | with romfs, it needed 3079 blocks. |
18 | |
19 | To create such a file system, you'll need a user program named |
20 | genromfs. It is available on http://romfs.sourceforge.net/ |
21 | |
22 | As the name suggests, romfs could be also used (space-efficiently) on |
23 | various read-only media, like (E)EPROM disks if someone will have the |
24 | motivation.. :) |
25 | |
26 | However, the main purpose of romfs is to have a very small kernel, |
27 | which has only this filesystem linked in, and then can load any module |
28 | later, with the current module utilities. It can also be used to run |
29 | some program to decide if you need SCSI devices, and even IDE or |
30 | floppy drives can be loaded later if you use the "initrd"--initial |
31 | RAM disk--feature of the kernel. This would not be really news |
32 | flash, but with romfs, you can even spare off your ext2 or minix or |
33 | maybe even affs filesystem until you really know that you need it. |
34 | |
35 | For example, a distribution boot disk can contain only the cd disk |
36 | drivers (and possibly the SCSI drivers), and the ISO 9660 filesystem |
37 | module. The kernel can be small enough, since it doesn't have other |
38 | filesystems, like the quite large ext2fs module, which can then be |
39 | loaded off the CD at a later stage of the installation. Another use |
40 | would be for a recovery disk, when you are reinstalling a workstation |
41 | from the network, and you will have all the tools/modules available |
42 | from a nearby server, so you don't want to carry two disks for this |
43 | purpose, just because it won't fit into ext2. |
44 | |
45 | romfs operates on block devices as you can expect, and the underlying |
46 | structure is very simple. Every accessible structure begins on 16 |
47 | byte boundaries for fast access. The minimum space a file will take |
48 | is 32 bytes (this is an empty file, with a less than 16 character |
49 | name). The maximum overhead for any non-empty file is the header, and |
50 | the 16 byte padding for the name and the contents, also 16+14+15 = 45 |
51 | bytes. This is quite rare however, since most file names are longer |
52 | than 3 bytes, and shorter than 15 bytes. |
53 | |
54 | The layout of the filesystem is the following: |
55 | |
56 | offset content |
57 | |
58 | +---+---+---+---+ |
59 | 0 | - | r | o | m | \ |
60 | +---+---+---+---+ The ASCII representation of those bytes |
61 | 4 | 1 | f | s | - | / (i.e. "-rom1fs-") |
62 | +---+---+---+---+ |
63 | 8 | full size | The number of accessible bytes in this fs. |
64 | +---+---+---+---+ |
65 | 12 | checksum | The checksum of the FIRST 512 BYTES. |
66 | +---+---+---+---+ |
67 | 16 | volume name | The zero terminated name of the volume, |
68 | : : padded to 16 byte boundary. |
69 | +---+---+---+---+ |
70 | xx | file | |
71 | : headers : |
72 | |
73 | Every multi byte value (32 bit words, I'll use the longwords term from |
74 | now on) must be in big endian order. |
75 | |
76 | The first eight bytes identify the filesystem, even for the casual |
77 | inspector. After that, in the 3rd longword, it contains the number of |
78 | bytes accessible from the start of this filesystem. The 4th longword |
79 | is the checksum of the first 512 bytes (or the number of bytes |
80 | accessible, whichever is smaller). The applied algorithm is the same |
81 | as in the AFFS filesystem, namely a simple sum of the longwords |
82 | (assuming bigendian quantities again). For details, please consult |
83 | the source. This algorithm was chosen because although it's not quite |
84 | reliable, it does not require any tables, and it is very simple. |
85 | |
86 | The following bytes are now part of the file system; each file header |
87 | must begin on a 16 byte boundary. |
88 | |
89 | offset content |
90 | |
91 | +---+---+---+---+ |
92 | 0 | next filehdr|X| The offset of the next file header |
93 | +---+---+---+---+ (zero if no more files) |
94 | 4 | spec.info | Info for directories/hard links/devices |
95 | +---+---+---+---+ |
96 | 8 | size | The size of this file in bytes |
97 | +---+---+---+---+ |
98 | 12 | checksum | Covering the meta data, including the file |
99 | +---+---+---+---+ name, and padding |
100 | 16 | file name | The zero terminated name of the file, |
101 | : : padded to 16 byte boundary |
102 | +---+---+---+---+ |
103 | xx | file data | |
104 | : : |
105 | |
106 | Since the file headers begin always at a 16 byte boundary, the lowest |
107 | 4 bits would be always zero in the next filehdr pointer. These four |
108 | bits are used for the mode information. Bits 0..2 specify the type of |
109 | the file; while bit 4 shows if the file is executable or not. The |
110 | permissions are assumed to be world readable, if this bit is not set, |
111 | and world executable if it is; except the character and block devices, |
112 | they are never accessible for other than owner. The owner of every |
113 | file is user and group 0, this should never be a problem for the |
114 | intended use. The mapping of the 8 possible values to file types is |
115 | the following: |
116 | |
117 | mapping spec.info means |
118 | 0 hard link link destination [file header] |
119 | 1 directory first file's header |
120 | 2 regular file unused, must be zero [MBZ] |
121 | 3 symbolic link unused, MBZ (file data is the link content) |
122 | 4 block device 16/16 bits major/minor number |
123 | 5 char device - " - |
124 | 6 socket unused, MBZ |
125 | 7 fifo unused, MBZ |
126 | |
127 | Note that hard links are specifically marked in this filesystem, but |
128 | they will behave as you can expect (i.e. share the inode number). |
129 | Note also that it is your responsibility to not create hard link |
130 | loops, and creating all the . and .. links for directories. This is |
131 | normally done correctly by the genromfs program. Please refrain from |
132 | using the executable bits for special purposes on the socket and fifo |
133 | special files, they may have other uses in the future. Additionally, |
134 | please remember that only regular files, and symlinks are supposed to |
135 | have a nonzero size field; they contain the number of bytes available |
136 | directly after the (padded) file name. |
137 | |
138 | Another thing to note is that romfs works on file headers and data |
139 | aligned to 16 byte boundaries, but most hardware devices and the block |
140 | device drivers are unable to cope with smaller than block-sized data. |
141 | To overcome this limitation, the whole size of the file system must be |
142 | padded to an 1024 byte boundary. |
143 | |
144 | If you have any problems or suggestions concerning this file system, |
145 | please contact me. However, think twice before wanting me to add |
146 | features and code, because the primary and most important advantage of |
147 | this file system is the small code. On the other hand, don't be |
148 | alarmed, I'm not getting that much romfs related mail. Now I can |
149 | understand why Avery wrote poems in the ARCnet docs to get some more |
150 | feedback. :) |
151 | |
152 | romfs has also a mailing list, and to date, it hasn't received any |
153 | traffic, so you are welcome to join it to discuss your ideas. :) |
154 | |
155 | It's run by ezmlm, so you can subscribe to it by sending a message |
156 | to romfs-subscribe@shadow.banki.hu, the content is irrelevant. |
157 | |
158 | Pending issues: |
159 | |
160 | - Permissions and owner information are pretty essential features of a |
161 | Un*x like system, but romfs does not provide the full possibilities. |
162 | I have never found this limiting, but others might. |
163 | |
164 | - The file system is read only, so it can be very small, but in case |
165 | one would want to write _anything_ to a file system, he still needs |
166 | a writable file system, thus negating the size advantages. Possible |
167 | solutions: implement write access as a compile-time option, or a new, |
168 | similarly small writable filesystem for RAM disks. |
169 | |
170 | - Since the files are only required to have alignment on a 16 byte |
171 | boundary, it is currently possibly suboptimal to read or execute files |
172 | from the filesystem. It might be resolved by reordering file data to |
173 | have most of it (i.e. except the start and the end) laying at "natural" |
174 | boundaries, thus it would be possible to directly map a big portion of |
175 | the file contents to the mm subsystem. |
176 | |
177 | - Compression might be an useful feature, but memory is quite a |
178 | limiting factor in my eyes. |
179 | |
180 | - Where it is used? |
181 | |
182 | - Does it work on other architectures than intel and motorola? |
183 | |
184 | |
185 | Have fun, |
186 | Janos Farkas <chexum@shadow.banki.hu> |
187 |
Branches:
ben-wpan
ben-wpan-stefan
javiroman/ks7010
jz-2.6.34
jz-2.6.34-rc5
jz-2.6.34-rc6
jz-2.6.34-rc7
jz-2.6.35
jz-2.6.36
jz-2.6.37
jz-2.6.38
jz-2.6.39
jz-3.0
jz-3.1
jz-3.11
jz-3.12
jz-3.13
jz-3.15
jz-3.16
jz-3.18-dt
jz-3.2
jz-3.3
jz-3.4
jz-3.5
jz-3.6
jz-3.6-rc2-pwm
jz-3.9
jz-3.9-clk
jz-3.9-rc8
jz47xx
jz47xx-2.6.38
master
Tags:
od-2011-09-04
od-2011-09-18
v2.6.34-rc5
v2.6.34-rc6
v2.6.34-rc7
v3.9